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12-03-2008, 12:11 PM   #31
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Alas, I'm starting to loose hope...

I just talked to a technician at Pentax. I left them with my K20D with a Katz Eye screen installed and a Pentax K 50mm f/1.2 lens. My complaint is that the combination front focuses. It doesn't matter if I use the split screen to focus or the AF indicator - the body front focuses.

So, first the technicial complains that it's not a digital lens. Ok, I say, it happens with all my other lenses too, DA*, ltd, Sigma, it doesn't matter - if I focus using the split prism, the picture turns out front focused. He said he tried his 20-40mm and it works just fine. I asked him to please check using manual focus, not AF, and he'll see there is a problem. He said ok, he'll take a look at it, but it sounded like he didn't know what he was going to do. I then suggested that Rachael Katz (owner of Katz Eye) suggested the shim for the manual focus housing likely needs to be replaced with one of a different thickness. He said ya, ya, he'd take a look at it, but in a doubtful kinda tone. Ugh... If I get it back with nothing done to it, I'm going to scream.

12-03-2008, 12:41 PM   #32
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QuoteOriginally posted by dkittle Quote
Alas, I'm starting to loose hope...

I just talked to a technician at Pentax. I left them with my K20D with a Katz Eye screen installed and a Pentax K 50mm f/1.2 lens. My complaint is that the combination front focuses. It doesn't matter if I use the split screen to focus or the AF indicator - the body front focuses.

So, first the technicial complains that it's not a digital lens. Ok, I say, it happens with all my other lenses too, DA*, ltd, Sigma, it doesn't matter - if I focus using the split prism, the picture turns out front focused. He said he tried his 20-40mm and it works just fine. I asked him to please check using manual focus, not AF, and he'll see there is a problem. He said ok, he'll take a look at it, but it sounded like he didn't know what he was going to do. I then suggested that Rachael Katz (owner of Katz Eye) suggested the shim for the manual focus housing likely needs to be replaced with one of a different thickness. He said ya, ya, he'd take a look at it, but in a doubtful kinda tone. Ugh... If I get it back with nothing done to it, I'm going to scream.
I've never know technicians to be happy w/ working with 3rd party parts..
bring your own oil and filter to your car dealer sometime
you also state an anomaly. The AF indicator lights when the lens is said to be in focus w/ the AF module which has absolutely nothing to do w/ the focusing screen... if it was just the screen then when AF locked (red spot) it would appear out of focus to your eyes...
IF they have a shim that works you should be fine....
7 is AF module, 4 is your screen.
12-03-2008, 01:13 PM   #33
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QuoteOriginally posted by jeffkrol Quote
I've never know technicians to be happy w/ working with 3rd party parts..
bring your own oil and filter to your car dealer sometime
you also state an anomaly. The AF indicator lights when the lens is said to be in focus w/ the AF module which has absolutely nothing to do w/ the focusing screen... if it was just the screen then when AF locked (red spot) it would appear out of focus to your eyes...
IF they have a shim that works you should be fine....
7 is AF module, 4 is your screen.
I've had Katz Eye screens on 3 of my Pentax DSLR cameras and they worked flawlessly. Beter then SAFOX AF indeed.
Katz Eye are used on every major SLR system, and although I can imagine that they can make a bad focusing screen once in a while, I've never herd about one with a wrong thickness.

This however can be measured easily with a presize caliper.
12-03-2008, 05:44 PM   #34
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QuoteOriginally posted by awo425 Quote
I've had Katz Eye screens on 3 of my Pentax DSLR cameras and they worked flawlessly. Beter then SAFOX AF indeed.
Katz Eye are used on every major SLR system, and although I can imagine that they can make a bad focusing screen once in a while, I've never herd about one with a wrong thickness.

This however can be measured easily with a presize caliper.
First, the light to the AF module NEVER passes through the screen. It is only for manual focus and would affect metering and MANUAL focus... see diagram.
As to Katz eye see the following thread.......
https://www.pentaxforums.com/forums/pentax-dslr-discussion/35884-front-focusi...n-curable.html
Katz eye confirmed the front-focus problem, swapped out the shim (was 0.35mm, now 0.25mm), and reinstalled the screen. Focus is now quite accurate -- if there's any error, it's too small for me to measure.


12-03-2008, 10:32 PM   #35
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Just an update:

It's noon here and I did some informal tests. Shots confirmed that in daylight, no adjustment was sharper than with adjustment.

So yup, my K20D front focuses only in tungsten. Fine in daylight and flourescent.

Sigh
12-06-2008, 11:10 AM   #36
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After reading this thread, I spent a couple of hours with the DIY Nikon focus chart. I noted no significant issues with front or rear focus in daylight across four lenses and two cameras. I found definite front-focussing issues under tungsten light. Conclusions:

1. I did my tests under two separate tungsten lamps, one a trilight. I concluded that front focussing is related to lighting wavelength, obviously, but not to lighting level. I did not test with fluorescent light.

2. Front focussing was present equally whether I used MF or AF.

3. Front focussing is directly related to aperture settings. I had no problem with any lens on any camera under any light at F4. At F2.8, I had slight front-focussing under tungsten light. At F1.4, I experienced severe front focussing.

My final conclusion after all this mucking around is that I am going to get much more familiar with my Sigma flash, Omni-bounce, Puffer, umbrella, and the K20's wireless flash mode. These tests will also influence my future lens selections. The allure of super wide apertures is much diminished for me.
12-06-2008, 02:36 PM   #37
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QuoteOriginally posted by audiobomber Quote
3. Front focussing is directly related to aperture settings. I had no problem with any lens on any camera under any light at F4. At F2.8, I had slight front-focussing under tungsten light. At F1.4, I experienced severe front focussing.
True, and this is largely due the shallower DOF at wider apertures - a slight focus error is going to be much more noticeable if it moves your intended focus point completely out of the zone of acceptable sharpness. and in the case of MF, this is compounded by the fact that the focus screen lies, showing you rather more DOF than will actually be there at wide apertures. So there will always be things in focus in the viewfinder that don't end up in focus in the picture, and on my camera at least, it's the area toward the *front* of the apparently-in-focus zone that end up OOF.

QuoteQuote:
The allure of super wide apertures is much diminished for me.
I tend to agree, and it's one reason I don't sweat my DA40 being "only" f/2.8. Much wider than that and DOF becomes shallow enough that nailing focus is tricky. But even if AF will always FF in tungsten, you *can* learn to MF correctly. It's simply a matter of learning to put the target toward the very *front* of the in-focus zone in the viewfinder, rather than towards the rear. Takes practice to learn to focus this way, but it can be done.
12-07-2008, 12:07 AM   #38
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QuoteOriginally posted by Marc Sabatella Quote
True, and this is largely due the shallower DOF at wider apertures - a slight focus error is going to be much more noticeable if it moves your intended focus point completely out of the zone of acceptable sharpness.
But I don't think there was a focus error Marc. I was using a tripod, 2s timer and a lot of care, and I had consistent misfocus, in AF and MF.

QuoteOriginally posted by Marc Sabatella Quote
But even if AF will always FF in tungsten, you *can* learn to MF correctly. It's simply a matter of learning to put the target toward the very *front* of the in-focus zone in the viewfinder, rather than towards the rear. Takes practice to learn to focus this way, but it can be done.
I suppose, but the trick is to figure out when the camera will lie to you, and how much to correct.

12-07-2008, 03:26 PM   #39
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QuoteOriginally posted by audiobomber Quote
But I don't think there was a focus error Marc. I was using a tripod, 2s timer and a lot of care, and I had consistent misfocus, in AF and MF.
I meant, on the part of the camera. If there is a FF or BF problem, it will indeed almost always show up only at wide apertures.

QuoteQuote:
I suppose, but the trick is to figure out when the camera will lie to you, and how much to correct.
Right. That's why I recommend doing some controlled testing with a focus test chart or a newspaper (for MF only!) where you can compare what you see in the viewfinder against what you see in the image. Won't take long before you realize that at any apertur wider than f/2.8, the viewfinder will show more in focus than will be in the picture, and you'll also get handle on *where* the discrepancy is. For me, it's always the things in the front of the apparently-in-focus zone that end up OOF.
12-07-2008, 09:15 PM   #40
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QuoteOriginally posted by jeffkrol Quote
First, the light to the AF module NEVER passes through the screen. It is only for manual focus and would affect metering and MANUAL focus... see diagram.
As to Katz eye see the following thread.......
https://www.pentaxforums.com/forums/pentax-dslr-discussion/35884-front-focusi...n-curable.html
Katz eye confirmed the front-focus problem, swapped out the shim (was 0.35mm, now 0.25mm), and reinstalled the screen. Focus is now quite accurate -- if there's any error, it's too small for me to measure.
just out of curiosity, did I say that "the light to the AF module passes through the screen"? Thanks for education though.

Looks like Pentax changes something with k20D cameras. istD and K10D did not require any shims, nor adjustments for Kat Eye screens to work properly. Well, there's always something new.
12-07-2008, 10:04 PM   #41
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K20s have BF/FF problems?

QuoteOriginally posted by dkittle Quote
Alas, I have the same problem with my K20D, it front focuses very badly with both AF and manual focus. I've currently got it out with Pentax for servicing, hoping that they can at least reshim the damn thing and fix the manual focus issue. For AF, I can dial in adjustment and get every lens working except for my FA50 1.4 - even at -10 it still front focuses quite badly. I've repeated the test 4 times now, always under tungstan.

I just bought a used K20D and it has some focus issues but not quite as bad. It front focuses with manual focus and is out a little either way with many AF lenses. The AF doesn't bother me that much as we can correct it (most times), its the manual focus being out that bothers me most.

I might go retry this test in daylight and see what results I get.

Interesting the main reason for me feeling like I "should" upgrade to the K20 from my GX10 is the ability to set FF/BF for each lens. I know the K10s have lots of issues and it seems sooner or latter every owner will have a problem. I just got my GX10 replaced a month ago (it stopped working) and it has superior AF compared to my old, I get no OOF shots with any of my 4 lens. Before I was using the firmware hack. Now because every shot is sharp and in focus I don't even bother to check for any BF/FF. But I feel one day it will again (BF/FF) but if I upgrade to the K20 that day will never come. True or not true, or is the truth in the middle?
12-08-2008, 01:44 AM   #42
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A number of my friends who had weird focusing problems on the K10 and moved to a K20 had the problems disappear. I haven't had the need to adjust my focusing, all I have is that funky focusing in tungsten light. Everywhere else it's perfect on all 4 lenses I've had (3 DA limiteds and 16-50).
12-08-2008, 04:22 PM   #43
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QuoteOriginally posted by dkittle Quote
My lenses needed both positive and negative adjustments.
This is one of the "great lens mysteries". Unfortunately, no one seems to have the answer.

QuoteOriginally posted by RiceHigh Quote
Forget any "45 degree" testing
What is the rationale for this statement? Samsung/Pentax itself uses this approach when calibrating cameras, one (immaterial) difference being the angle of 30 instead of 45 degrees. The other difference is that their black to white transition is in parallel to the camera sensor, but frankly I don't see how you want to distinguish between a line (transition) being viewed at an angle or not. Check page 121 of the Samsung GX 10 service manual.

QuoteOriginally posted by jeffkrol Quote
...I doubt that, contrary to a few naysayers, this is a Pentax only problem...
Thanks for posting these user testimonies. I would have been surprised if Pentax had the only AF system exhibiting this problem and not doing anything about it. Apparently it is a problem of all prism-based AF systems and only some models seem to implement countermeasures (taking light temperature into account).

Last edited by Class A; 01-09-2009 at 05:48 AM.
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